The invention relates to an arrangement for generating RF fields in the examination volume of an MR apparatus, which arrangement includes a body coil which consists of a plurality of resonator segments which are arranged around the examination volume and each of which consists of at least one conductor element which extends parallel to the longitudinal axis of a main field magnet and of at least one capacitor element.
The invention also relates to an MR apparatus which includes an arrangement for generating RF fields in accordance with the invention.
The RF system of a conventional MR apparatus includes a transmission and receiving coil such as, for example, an integrated body coil which can be used for the volume imaging of the examination volume. In order to achieve an enhanced receiving quality (enhanced signal-to-noise ratio, higher resolution), separate surface coils or so-called phased array coils can also be used. The body coils used for the excitation as well as the detection of MR signals are usually so-called birdcage resonators (birdcage coils). These resonators consist of a plurality of conductor rods which are arranged around the examination volume so as to extend parallel to the main field direction, said conductor rods being connected to one another via loop conductors at the extremities of the coil. The resonance behavior of the body coil is determined by capacitor elements which connect the conductor elements in a manner such that they form a network. The first resonance mode (fundamental mode) of these resonators is characterized by a B1 field distribution which is uniform throughout the inner zone of the resonator. The same holds for the spatial sensitivity profile in the detection mode. Therefore, for volume imaging the body coil customarily operates in the fundamental mode during the transmission and the reception. It is also possible to control the resonator in such a manner that orthogonal resonances are excited at the same frequency, that is, decoupled resonances intended for quadrature detection.
In MR imaging the nuclear magnetization is localized within the examination volume by means of temporally variable, spatially inhomogeneous magnetic fields (magnetic field gradients). For imaging the spin resonance signal is recorded in the time domain as a voltage which is induced into the body coil surrounding the examination volume under the influence of a suitable sequence of RF and gradient pulses. The actual image reconstruction then takes place by Fourier transformation of the time signals. The sampling of the reciprocal k space, governing the volume to be imaged (field of view or FOV) as well as the image resolution, are determined by the number, the distance in time, the duration and the strength of the gradient pulses used. Requirements imposed on the image format and the image resolution govern the number of phase encoding steps and hence the duration of the imaging sequence. In contemporary MR apparatus the aim is to realize imaging with an as high as possible resolution in an as short as possible period of time. Special requirements are thus imposed as regards the gradient system of the MR apparatus in order to enable as fast as possible switching of as strong as possible magnetic field gradients.
The gradient system in MR apparatus is customarily accommodated in a so-called gradient tube which surrounds the examination volume. Between the gradient tube and the body coil there is arranged an RF shield which on the one hand keeps interference signals from the surroundings away from the examination volume and on the other hand prevents radiation of RF power to the surroundings of the MR apparatus. The gradient field strength can be increased, for example, by reducing the diameter of the gradient tube. When the examination volume and the inner diameter of the body coil are maintained in such a case, the distance between the RF shield and the body coil will be reduced. It is a drawback that the small distance between the conductor elements of the body coil and the RF shield leads to a reduction of the B1 field strength or the detection sensitivity in the interior of the coil. This has the drawback that the transmission power required is increased and at the same time the signal-to-noise ratio is degraded. The reduction of the tube diameter also has adverse effects on the resonance behavior of the body coil, because the frequency spacing of the individual resonance modes becomes too small, so that undesirable mode coupling occurs. When customary birdcage resonators are used as the transmission and receiving coil, therefore, limits are imposed in respect of increasing the gradient field strength by reducing the tube diameter.
Alternatively, the gradient field strength can be increased by utilizing a gradient tube which has an asymmetrical cross-section. However, this also necessitates the use of asymmetrical body coils. Such asymmetrical resonator arrangements give rise to a series of problems in practice. On the one hand, it is difficult to orthogonalize such a body coil so as to enable quadrature operation. On the other hand, the RF field variation inside the coil is dependent on the current distribution in the array of conductor elements, so that the homogeneity of the RF field in the transmission mode, and hence of the spatial sensitivity profile in the receiving mode, usually is not satisfactory in the case of asymmetrical resonators.
Leussler et al. already proposed a birdcage resonator which operates in a degenerated resonance mode in which the spatially neighboring segments of the resonator are decoupled; that is, they oscillate independently of one another (see Leussler et al., Proceedings of the ISMRM, No. 176, Vancouver 1997). In the known coil the resonance behavior employed is achieved by a suitable choice of the capacitor elements. Unfortunately, only the decoupling of directly neighboring resonator segments can be achieved in this way. The coupling between more remote segments is too high, so that operation of the coil in the resonance transmission mode is not possible.